At the heart of a digital slr is its sensor. ...... In marketing terms each of the sensors will have been proven at the higher end and then implemented at a
more manageable price point at the lower end. Quite clever really and
in production terms optimises investment in sensor technology. Canon,
on the other hand, appear to design a sensor per camera...
Tony

I agree that the having common sensors across several categories will give more economies of scale. However, when amateurs with intermediate telephoto lenses shoot with crop cameras they can get that longer reach. If your volumes are good for each sensor, then it makes sense to have a different sensor. Canon has to decide if it fits into their strategy. And I think Canon will continue to have multiple sensors in the medium term.